A capacitor is an electrical device that can store energy in the electric field between a pair of closely spaced conductors. When voltage is applied to the capacitor, electrical charges of equal magnitude, but opposite polarity, build up on each plate. It is known by fundamental physical law that the capacitance of a capacitor is determined by the surface area of the parallel plates (C≈εA/d, A is surface area). Capacitors are used in electrical circuits as energy-storage devices.
A supercapacitor (or ultracapacitor) is an electrochemical double layer capacitor that has usually high capacitance when compared to common capacitors. The capacitance of a supercapacitor is established by the electrochemical double layer. The physical mechanism of the electrochemical double layer has been elaborated in previous literature such as: Conway, B. E., Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications (2004); Burke, A., Ultracapacitors: Why, How, and Where Is The Technology. Journal of Power Sources 91, 37-50 (2000); and Pandolfo, A. G. & Hollenkamp, A. F., Carbon Properties And Their Role In Supercapacitors. Journal of Power Sources 157, 11-27 (2006).
The supercapacitor's high capacitance comes from using conductive materials of very high specific surface area. Most conventional supercapacitors use active carbon fiber cloth, active carbon particles or carbon fibers as electrode materials. These carbon materials are used because they have high theoretical specific surface area (˜1000-2000 m2/g). Correspondently, the calculated capacitance per surface area for active carbon materials is about 10-15 μF/cm2. Therefore, supercapacitors using active carbon material as electrodes have much higher capacitance (100-300 F/g) compared to common capacitors (on the order of μF or pF). With such high capacitance, supercapacitors are used as energy storage devices. Because supercapacitors have much less internal resistance than batteries, they are particularly suitable for providing transient power, for example providing cold-cranking pulse power in electrical vehicles.
The energy stored in a capacitor is calculated by (½)CV2. The maximum power that a capacitor can output is V2/(4R). R is the equivalent series resistance. It is known that conventional supercapacitors have a working voltage of ˜3V.
According to Pandolfo and Hollenkamp, the carbon supercapacitor has specific energy of 1-10 Wh/kg and specific power of 0.5-10 KW/kg. In order to improve the performance of supercapacitors, advanced materials should be used as electrode materials.
Two factors limit the capacitance of superconductors—the pore distribution and the resistance of the electrode material. Electrolytes cannot access pores less than 2 nm in diameter. Small pores limit the capacitance of the electrode material. To improve the performance of the active carbon material used as electrodes, the pore distribution of the material should be optimized. The resistance of electrodes is another constraint that limits the specific power of supercapacitors. To increase the specific power of supercapacitors, both the bulk resistance and interface resistance of electrode materials should be rendered very small.